A number of devices of varying effectiveness and utility have been proposed to mitigate the effects of boundary layer separation or laminar-to-turbulent transition. These devices have generally employed mechanical means and have been directed to reenergizing a stagnant boundary layer. Such devices include wing vortex generators, wing leading and trailing edge slats and slots, boundary layer air suction mechanisms, and boundary layer air blowers.
Fixed vortex generators were employed on the upper surfaces of the swept wings of early jet transports to create controlled vortices that would reenergize and narrow the stagnant boundary layer that developed near the trailing edge, thereby preventing boundary layer separation from the wing. Although the reattached boundary layer was turbulent, it was preferable to separation and required no additional energy input to function. Another partial solution to the problem employed various forms of blowers designed to reenergize and maintain laminar flow by injecting high energy jets of smooth laminar air at critical regions upon the aerodynamic surface whose boundary layer was to be controlled. Disadvantages to this method included weight penalties due to necessary air ducting and engine power loss associated with the use of compressor bleed air to produce high energy air jets. Where independent pumps were used to create the high energy air jets, additional penalties of weight and complexity were encountered. A third approach involves the removal of low energy boundary layer air through suction slots or porous surfaces on the airfoil. Most of the work in laminar flow control has concentrated on the use of fluid suction to remove the boundary layer through slots in the surface of an aircraft wing. As the stagnant layer is removed from the surface of the airfoil, it is replaced by higher energy air from higher levels in the boundary layer. Fluid suction systems, as with blower systems, suffer the drawbacks of being complex, relatively heavy, and not particularly energy efficient. In addition, the requirements for internal airfoil ducting and air pumps may prohibit the use of blower or suction systems on some aerodynamic members for which boundary layer control might otherwise be desirable. Slotted leading edge slats and slotted trailing edge flaps utilize aerodynamic pressure to reenergize stagnant boundary layers, and are widely used in high lift airfoil configurations such as landing or takeoff. As these devices are primarily associated with temporary airfoil reconfigurations of limited duration, their suitability for high altitude, high speed cruise conditions of long duration is marginal.